1. Field of the Invention
The present invention relates to a coating apparatus, a method for producing an electrophotographic photosensitive member using the coating apparatus, and a method for mass-producing electrophotographic photosensitive members.
2. Description of the Related Art
An electrophotographic photosensitive member typically has a support and a photosensitive layer formed on the support. Also, an electrically conductive layer, an under coat layer (intermediate layer), and the like may be provided between the support and the photosensitive layer, and a protective layer may be provided on the photosensitive layer.
For producing an electrophotographic photosensitive member, methods for forming a layer such as a photosensitive layer on a support include, for example, a dip coating method, a roll coater method, a spray method, an electrostatic coating method, and the like. Among these, the dip coating method may be advantageous where an object to be coated has a three-dimensional shape, such as a cylindrical shape, a seamless belt shape, or the like. In addition, the dip coating method is advantageous in mass production because a single coating apparatus (dip coating apparatus) is capable of simultaneously coating a plurality of objects. Thus, the dip coating method has been widely employed for the production (mass production) of electrophotographic photosensitive members.
FIG. 6 shows an example of a coating apparatus (dip coating apparatus).
In the coating apparatus shown in FIG. 6, a coating liquid 20 is sent to the lower portion of a coating bath 9 via a collection tank 2 and a filter 4 with the aid of a liquid transport unit 3 such as a pump or the like. Also, the coating liquid 20 in excess of the capacity of the coating bath 9 falls into an overflow vessel 10, and then is sent to the collection tank 2 via piping. The reference numeral 6 indicates the liquid surface of the coating liquid 20. Using the coating liquid circulator described above, the coating liquid 20 circulates within the coating apparatus. Also, a cover cap 7 that is provided with a through port 8 for the passage of an object to be coated 1 therethrough covers the coating bath 9. The cover cap 7 suppresses the entry of foreign substances into the coating liquid 20 and the solvent volatilization from the coating liquid 20. The object to be coated/coated object 1 is partially gripped by a lowering and lifting unit (not shown), is dipped into the coating liquid 20 contained in the coating bath 9, and then lifted up therefrom, whereby a coating film (wet coating film) is formed on the surface of the object to be coated 1. Also, a hood 5 prevents the coating film from being affected by the ambient conditions after being lifted out. The hood 5 is installed above the through port 8 of the cover cap 7.
During intervals of producing an electrophotographic photosensitive member or the like, the circulation of the coating liquid within the coating apparatus may be temporarily stopped in order to exchange a filter at regular intervals or maintain a lowering and lifting unit and a liquid transport unit. While the circulation of the coating liquid within the coating apparatus is stopped, a solvent is evaporated from the liquid surface of the coating liquid in the coating bath, resulting in an increase in the viscosity of the coating liquid. Consequently, a partially semi-solid film (hereinafter referred to as “semisolid film”) may be formed on the liquid surface of the coating liquid. When the circulation of the coating liquid is resumed, a semisolid film is caught by, and stays at, the upper-end edge portion of the coating bath, resulting in the retention thereof. A semisolid film remaining on the upper-end edge portion of the coating bath makes the flow of the coating liquid in the coating bath (the overflow of the coating liquid from the coating bath) non-uniform, whereby coating defects such as unevenness in film thickness or the like may occur.
As a method for suppressing the retention of foreign substances at the liquid surface of the coating liquid in the coating bath, Japanese Patent Laid-Open No. 2002-323778 discloses a method for providing a notch part or a stop part at the upper-end portion of the coating bath.
Also, as a method for maintaining a uniform overflow of the coating liquid from the coating bath, Japanese Patent Laid-Open No. 07-132258 discloses a coating bath in which a sloped surface 14, as shown in FIGS. 8A and 8B, sloping toward outside from a upper-end surface 12 is provided relative to the shape of the typical coating bath as shown in FIGS. 7A and 7B.
However, the method disclosed in Japanese Patent Laid-Open No. 2002-323778 is effective for air bubbles but substantially ineffective for semisolid films. Furthermore, the flow of the coating liquid near the notch part or the stop part becomes non-uniform, resulting in the occurrence of coating defects.
Also, the coating bath disclosed in Japanese Patent Laid-Open No. 07-132258 may reduce the risk of the retention of the semisolid film to some extent, but still is not sufficient for suppressing the retention of the semisolid film.